1. Field of the Invention
The present invention relates to a semiconductor storage device including FBCs (Floating Body Cells), each of which stores data by storing majority carriers in a channel body of a FET on a semiconductor layer, which is formed on a substrate via an insulation film.
2. Background Art
As for a conventional DRAM cell including one transistor and one capacitor having a trench capacitor or a stacked capacitor, there is a concern that its fabrication becomes difficult as it becomes finer. As a memory cell that can replace the DRAM cell of this kind, a new memory cell FBC, which stores information by storing majority carriers in a floating body of a FET formed on SOI (Silicon on Insulator) or the like, was proposed (see Japanese Patent Application Laid-Open Publication Nos. 2003-68877 and 2002-246571).
Since the FBC is a non-destructive read-out cell, cell data is not destroyed even if a word line is activated. Therefore, it is not necessary to dispose, for each bit line, a sense amplifier for sensing and amplifying data and writing the data back into a cell, but it suffices to dispose one sense amplifier for a plurality of bit lines. As a result, it can make the cell efficiency higher than the conventional DRAM.
However, “a scheme in which the number of sense amplifiers is less than the number of bit lines” means that the number of cells that can be refreshed simultaneously at the time of refresh is small. Therefore, the semiconductor storage device having the conventional sense amplifier for the same refresh cycle has a problem that the refresh busy rate is lowered as compared with the conventional DRAM. In other words, even if the retention time of the FBC cell can be made equal to the retention time of the conventional DRAM cell, memory refresh operation must be conducted in the FBC more frequently than in the conventional DRAM. This results in a problem that the ratio of time during which the read/write operation of the FBC can be arbitrarily conducted decreases as compared with the conventional DRAM.